Terrestrial and aquatic ecosystem responses to early Holocene rapid climate change (RCC) events in the South Carpathian Mountains, Romania

As demonstrated by an increasing number of palaeoclimatic and palaeoecological studies, rapid climate change events (RCCs) occurred frequently in the Holocene and their timing correlates well in the European records. Changes in vegetation composition and environmental conditions were significant during these RCC events. In this study we use high resolution pollen, stomata, micro- and macrocharcoal, macrofossil, siliceous algae, biogenic silica and organic content analyses from two alpine lake sediments (Lake Brazi, 1740 m a.s.l.; Lake Gales, 1990 m a.s.l.) in the Retezat Mts, South Carpathian Mountains, Romania. Our aim is to study ecosystem responses to RCCs between 12,000 and 7000 cal yr BP using high-resolution proxy analyses of the relevant sediment sections. We detected several significant changes in the terrestrial vegetation composition and aquatic ecosystems in case of both lakes. Complex ecosystem responses were found in connection with the early Holocene RCC intervals. Most prominently, the 10.2 ka climatic change likely fostered the extinction of Larix decidua from the shore of Lake Brazi, while climatic change during the 8.2 ka event facilitated the establishment of Carpinus betulus in the lower deciduous mixed oak forests, due to decreasing growing season temperatures, frequent summer droughts and associated recurrent fire events. Taken together, most of the significant pollen compositional changes reflected the periodic spread of pioneer deciduous tree taxa (mainly Fraxinus excelsior and Corylus avellana) during the early Holocene RCCs. In all cases, this change was connectable to increased regional fire activity and the temporary increase of herbs. The most significant change in the lake-ecosystems was often the short-lived spread of various planktonic diatom species, mainly the members of genus Aulacoseira. Sudden appearance and large-scale percentage increase of these taxa suggested higher water-depth and/or intensified water turbulence.